1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display and fabrication method thereof in which a fabrication process of the liquid crystal display is simplified.
2. Description of the Related Art
Generally, a Braun tube (or cathode ray tube: CRT) has been used most widely among displays for displaying image information until now. However, the CRT causes many inconveniences due to its large size and weight.
Accordingly, a flat panel display that can be used anywhere with ease due to its characteristic having a large display area and a thin thickness at the same time was developed. This flat panel display is replacing the Braun tube display. Especially, a liquid crystal display (LCD) shows characteristics to have an excellent resolution compared with other flat panel displays and a response speed as fast as the CRT in displaying a moving picture.
As well known to those skilled to the art, the operation principle of the LCD is based on the optical anisotropy and polarization property of liquid crystal molecules. Since liquid crystal molecules have a thin elongated structure, it is possible to control the alignment direction of the liquid crystal molecules by artificially applying an electric field.
Hence, if the alignment direction is adjusted arbitrarily, light can be transmitted or shielded according to the alignment direction of liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, so that the colors and images can be displayed.
Generally, in the liquid crystal display, a first substrate (for example, a thin film transistor substrate) and a second substrate (for example, a color filter substrate) are provided and attached to each other with a predetermined interval therebetween called a cell gap.
Here, the liquid crystal display will be described in more detail. The first substrate (e.g., thin film transistor substrate) includes gate lines and data lines formed on a transparent substrate in a matrix configuration.
A thin film transistor (TFT) functioning as a switching device is formed at each of the cross points of the gate lines and the data lines. A rectangular pixel electrode contacted with the drain electrode of the TFT is formed on a pixel region defined by a pair of gate lines and a pair of data lines.
The second substrate (e.g., color filter substrate) is provided on the other side facing with the transparent substrate on which a plurality of pixel electrodes are formed, includes a black matrix, a color filter layer, and a common electrode formed on a transparent substrate.
If a voltage is applied to one gate line and one data line of the liquid crystal display configured as described above, the thin film transistor (TFT) to which the voltage is applied is turned on. Accordingly, charges are accumulated on the pixel electrode connected to the drain electrode of the turned-on TFT and the alignment of liquid crystal molecules provided between the first substrate and the second substrate is changed.
In the substrate attachment process, the first substrate and the second substrate are attached to form a predetermined cell gap between the first substrate and the second substrate. Subsequently, liquid crystal is injected into an inner space between the first substrate and the second substrate. Thereafter, a process for electrically connecting the first substrate with the second substrate is performed. In this process, sealant, spacer and conductive material are used.
The sealant is a thermosetting resin such as an epoxy resin, and is formed as a seal line along an edge of the first substrate or the second substrate. Here, the sealant serves to attach the two substrates to each other and prevent external materials from being introduced into the liquid crystal layer injected into the cell gap between the two substrates.
The spacer is made of a material such as a plastic ball or a glass fiber and positioned inside the seal line to maintain the interval between the two adhered substrates constantly.
The conductive material is a conductor such as nickel or silver, and is positioned inside the seal line to connect a pad led from the common electrode of the first substrate with the second substrate electrically.
The liquid crystal panel constituting the liquid crystal display will be described.
FIG. 1 is a cross-sectional view exemplarily showing a liquid crystal panel employed to a liquid crystal display according to a related art.
As shown in FIG. 1, an upper substrate 112, such as a color filter substrate, faces with and is spaced from a lower substrate 114, such as an array substrate, by a predetermined distance. Liquid crystal is filled between the upper substrate 112 and the lower substrate 114.
Here, a gate electrode 118 is formed on the transparent substrate 102, such constituting the lower substrate 114, and a gate insulating layer 120 is formed on the entire surface of the transparent substrate including the gate electrode 118. A semiconductor layer 122 including an active layer 122a and an ohmic contact layer 122b is formed on the gate insulating layer 120. A source electrode 124 and a drain electrode 126 are formed on the semiconductor layer 122. A passivation layer 128 including a contact hole 130 is formed on a resultant substrate including the source electrode 124 and the drain electrode 126. A pixel electrode 132 is formed to contact with the drain electrode 126 through the contact hole 130. The pixel electrode 132 behaves one side electrode that applies voltage to the liquid crystal 116.
Here, the gate electrode 118, the semiconductor layer 122, the source electrode 124 and the drain electrode 126 constitute a thin film transistor T.
At a lower portion of a transparent substrate 101 constituting the upper substrate 112, a black matrix 134 is formed at the position corresponding to the thin film transistor T. The R, G, B color filters 136 are formed at the position corresponding to the pixel electrode 132. An overcoat layer 138 is formed at the lower portion of the black matrix 134 and R, G, B color filters 136. A common electrode 140 serving as another electrode is formed at the lower portion of the overcoat layer 138 to apply an electric field to the liquid crystal.
A spacer 142 is placed in an inner space between the common electrode 140 and the pixel electrode 132 to maintain the cell gap that is a distance between the upper substrate 112 and the lower substrate 114 constantly.
A seal pattern 144 is formed at an edge of a liquid crystal display panel 100 to maintain the cell gap constantly like the spacer 142 and attach the upper substrate 112 and the lower substrate 114.
Hereinafter, a fabrication process (such as a 5-mask process) of a lower substrate of a liquid crystal display will be exemplarily described.
First, a predetermined metal layer is deposited on a transparent substrate 102 and patterned using a first mask to form gate lines and gate electrodes 118.
Next, a gate insulating layer 120, an amorphous silicon layer and an impurity-contained amorphous silicon layer are deposited sequentially, and an active layer 122a and an impurity semiconductor layer 122 are formed by a photolithography process using a second mask.
Subsequently, a predetermined metal layer is deposited and patterned using a third mask to form data lines, source electrode 124 and drain electrode 126. Then, the impurity semiconductor layer exposed between the source electrode 124 and the drain electrode 126 is etched to form an ohmic contact layer 122b. 
A passivation layer 128 is deposited and patterned using a fourth mask to form a contact hole 130 that exposes a predetermined area of the drain electrode 126.
The passivation layer 128 is formed to prevent a thin film transistor from being damaged or degenerating due to scratch and moisture penetration in a rubbing process of the liquid crystal cell process of the liquid crystal display. The passivation layer 128 is made of silicon nitride (SiNx) or BCB (BenzoCycloButene) that is a kind of organic insulators, etc.
Finally, a transparent conductive material is deposited and patterned using a fifth mask to form a pixel electrode 132 made of transparent conductive material electrically connected to the drain electrode 126 through the contact bole 130.
As this transparent conductive material, indium Tin Oxide (ITO) is mainly used, which has a low contact resistance with metal in a subsequent process or a TAB bonding for the connection with to an external circuit.
The liquid crystal display configured as described above is prepared to print an alignment layer on the upper substrate and the lower substrate and perform a rubbing process so that liquid crystal to be injected is aligned in a predetermined direction. The seal pattern is printed on the upper substrate. The spacers are dispensed on the lower substrate so as to secure a predetermined inner space. A predetermined amount of conductive material is coated on a predetermined position so as to electrically connect the upper substrate and the lower substrate with each other. In general, the conductive material is dotted on an inner area of the printed seal pattern.
FIG. 2 is a schematic view illustrating that the upper substrate is electrically connected to the lower substrate by a conductive material in a liquid crystal display which the substrate attaching process is completed according to a related art.
Referring to FIG. 2, a conductive material 220 is formed by a predetermined amount on a predetermined position of a lower substrate 200, and is coved by to an upper substrate 210. After a TAB boding process, a Vcom terminal 230 of the lower substrate 200 is connected to a common electrode 140 of the upper substrate 210.
In the stack structure of the liquid crystal display manufactured by this fabrication method, the Vcom terminal 230 to be connected to the upper substrate electrically through the conductive material is formed exposed at the uppermost layer of the lower substrate. Accordingly, the conductive material is coated on the exposed Vcom terminal 230 of the lower substrate 200 during the attaching process so that the signal Vcom terminal 230 of the lower substrate 200 can be connected to the common electrode 140 of the upper substrate 210 easily.
On the other hand, researches for substantially reducing the fabrication cost by simplifying the fabrication processes of the liquid crystal display are actively being carried out. In this situation, it is strongly requested to invent a novel fabrication process in which the process for forming a contact hole to connect the pixel electrode to the drain electrode can be omitted.